1. Field of the Invention
The present invention relates to a method and apparatus useful for detecting total reduced sulfur in a sample gas. More particularly, it relates to a method and apparatus for detecting total reduced sulfur in which the sample gas is heated to oxidize the total reduced sulfur to SO.sub.2 and the SO.sub.2 content of the sample is measured prior to and after the heating step.
2. Discussion of the Prior Art
The total reduced sulfur content of a gas includes sulfur vapor, hydrogen sulfide, mercapto compounds, other organic compounds containing sulfur, and a variety of other substances; all of which are considered pollutants by the Environmental Protection Agency. It is known that these substances can be oxidized to SO.sub.2 by contacting them with oxygen at elevated temperatures. It is also known that SO.sub.2 absorbs radiation of a wavelength between 250 and 330 nm. Despite this knowledge, however, no simple, sensitive effective method or apparatus has been developed to measure the total reduced sulfur content of a gas, independent of its SO.sub.2 content.
The reason for this is simply that unless a reference reading is taken prior to heating the sample gas, there is no way to distinguish between the total reduced sulfur and the SO.sub.2 content of the sample gas. Such a reference reading is difficult to make because the conditions necessary for the reference reading are significantly different from those necessary for the total reduced sulfur reading. For one thing, the total reduced sulfur reading must be made at an elevated temperature, whereas the reference measurement must be made at a much lower temperature. Furthermore, unless oxygen is actually added to the sample gas, complete conversion of the total reduced sulfur components of the sample gas may not occur, particularly at high total reduced sulfur concentration.
Dual flow cell techniques have been used to avoid this problem, but the new problem of balancing the flows to each cell in such systems has been such that the sensitivity and accuracy of the dual cell system is lower than required for many applications. Unless a single cell, in situ method is used, highly accurate total reduced sulfur measurement can not be made without extreme difficulty, particularly on sample gases containing low total reduced sulfur concentrations in the presence of high SO.sub.2 concentration. Because of the temperature requirements placed on single cell techniques, however, in situ techniques have been considered too slow and have been avoided.
It has now been found that an in situ measurement is practical; that excess oxygen can be added to a closed, in situ system; and that the temperature of the receptacle in which the sample gas is contained can be cycled rapidly enough to make repeditive measurements of the total reduced sulfur content of a sample by first taking a reference measurment, adding an oxygen containing gas, then heating the gas to the elevated temperature, and finally, cooling the system so that the entire process can be repeated within a short time interval. It has also been found that such a cyclic process yields accurate results free of contamination by the previously measured sample gas. The high temperature and the consequent conversion of the reduced sulfur components to SO.sub.2 appears to have a self-cleaning effect on the system.